There are numerous examples in the petroleum and chemical industry where a gas/liquid mixture has to be separated. In many applications the mixture is a gas with a relatively small percentage of liquid that has to be removed from the gas. The liquid may be entrained in smaller or larger quantities by the flowing gas, normally less than 10 vol %, and typically 1-5 vol %. The liquid can be present in the form of small or large droplets, suspended in the form of a mist, as a froth, etc. Well-known examples are water and liquid hydrocarbons in natural gas, liquid absorption agents in a gas treated with such agents and lubricating oil or crude oil in hydrogen or hydrocarbon gas.
Generally, vertical vessels are used when a relatively small percentage of liquid is to be removed from a gas. An argument for the use of a vertical vessel for removing liquid from gas is that it occupies little area, which is particularly advantageous for offshore application on production platforms, but also in refineries.
It is further generally desired to minimize the size of the vertical vessel, for reasons of space requirement, but also in order to minimize capital cost of the separation vessel. However, reducing the vessel diameter means that the fluid velocities, in particular gas velocities, for a given throughput are increased. The internals of the vessel need to be able to provide sufficient separation efficiency for the required capacity at such high gas velocities.
It is customary to compare the capacity of gas/liquid separators (and fractionation columns) in terms of the so-called gas-load factor. The gas-load factor λ is defined as λ=Qg/Ag*(ρg/(ρl−ρg))1/2. Qg is the volumetric gas flow rate (m3/s), Ag is the vessel cross-sectional area (m2), ρg and ρl are the gas and liquid densities, respectively (kg/m3) (Souders and Brown, Ind. Engineering Chemistry, vol. 34 (1934) 98; Perry's Chemical's Engineers Handbook, 6th edition, McGraw-Hill, 1984, p. 18-6). The operating window of vertical gas/liquid separation vessels ranges from λ≦0.07 m/s for open separators without secondary separation internals or so-called demisters, to λ≧0.2 m/s, up to 0.25 m/s and even above for separators with secondary separation internals such as cyclones or vanepacks. The present invention is particularly advantageous in the case that the gas load factor λ is about 0.15 m/s or larger, in particular 0.2 m/s or larger. When a separator is used at a gas load factor above its design limit, the separation efficiency is lowered. Typically, for separators for use at gas load factors of 0.15 or larger, the separation efficiency needs to be 0.98 or higher, often 0.99 or higher, wherein the separation efficiency is defined as the fraction of the total liquid in the feed mixture that is removed and not present anymore in the outlet gas.
It is noted that the gas load factor in other separation devices can be much lower. For example, a horizontal three-phase (gas/liquid/liquid) separator is generally operated at gas load factors not exceeding ca 0.07 m/s, and in distillation columns gas load factors do not generally exceed about 0.12 m/s.
The gas/liquid mixture is generally admitted into a separator vessel using a so-called inlet device. Several types of inlet devices are known in the art. UK patent application with publication No. GB 036 606 and International Patent application with publication No. WO 03/033106 both describe a vertical separator with a tangential inlet device. These tangential inlet devices serve to introduce the mixture tangentially along the inner wall of the separation vessel, so as to set the mixture into a swirling motion in a separation zone of the vessel. Liquid separates in the separation zone due to centrifugal forces and runs down the inner vessel wall to accumulate in a liquid sump at the bottom. Above the liquid sump a baffle plate is mounted so that an annular opening with the inner vessel wall remains. The baffle plate demarcates the lower end of the separation zone, and separates the liquid sump from the swirling gas while allowing liquid to run down the inner vessel wall.
French Patent application with publication No. 2 780 659 discloses a vertical vessel into which the gas/liquid mixture is introduced from above. The mixture is distributed into a number of vertically downward streams symmetrically around the axis of the vertical vessel, by an upper baffle plate provided with openings. Each stream is passed down a helical wire separator that is vertically mounted in a separation compartment of the vessel, below the upper baffle plate. The wire separators have their outlets at their lower ends. The separation compartment is delimited from a lower liquid collection compartment by a lower deflector plate, onto which separated gas and liquid from each helical wire separator impinges directly in vertically downward direction. The plate is provided with a central opening through which liquid is discharged to a lower collection compartment. Gas is deflected upwardly and leaves the separation compartment sideways through the vessel wall.
The present invention relates to separation vessels provided with a vane inlet device, which serves to at least partially separate the gas/liquid mixture already at its admission into the vessel. A vane-type inlet device that is also referred to as a Schoepentoeter inlet device is for example described in British patent GB 1 119 699, and another vane inlet device is described in International Patent application publication No. WO 03/070348. A vane inlet device comprises a plurality of outwardly curved vanes arranged one behind the other in the direction of the inflowing gas/liquid mixture. Each vane curvedly extends with respect to the direction of inflowing mixture between an intercepting part having a leading edge substantially in or at a small angle with the direction of flow, and a deflecting part having an outwardly extending trailing edge.
The at least partially separated gas and liquid is admitted into the vessel through the outlet means of the separation device. In a vane-type inlet device, liquid is accumulated on the concave side of the vanes due to centrifugal forces. The outlet means is formed by the plurality of openings formed by consecutive vanes, wherein liquid leaves the vane tips from a layer at the concave side of the vanes, and gas leaves from the remainder of the openings. The liquid subsequently sinks towards the bottom of the vessel and the gas rises to the top.
Often, the gas/liquid separation device serves only for a primary separation at the vessel inlet, wherein in particular a further removal of entrained liquid from the gas is needed. To this end, one or more further separation devices can be arranged above the primary separation device, such as for example described in U.S. Pat. No. 4,767,424.
After the (pre-)separated gas and liquid have been admitted into the vessel via the outlet means of the vane inlet device, there is a risk of re-entrainment of liquid by the gas. It has been observed that this problem increases with increasing gas load factors, as they are encountered when trying to minimize vessel size for a given feed rate of gas/liquid mixture, or maximizing feed rate for a given vessel size. Gas and liquid come into contact again, particularly in the space directly below the (primary) separation device. It has been found that for gas-load factors .lambda. above 0.15 m/s, and in particular above 0.2 m/s re-entrainment of liquid due to high gas velocities in this space is a serious problem, because of loss of separation efficiency and consequently increased duty for secondary separators higher in the vessel.
Moreover it has been observed, that re-entrainment in this space can hinder or even prevent the accumulation of liquid in the liquid collection space, which is normally the space at the bottom of the vessel. Liquid outlet pipes from secondary separators can be arranged to debouche into the liquid collection space, wherein the outlet end of such a pipe is submerged in liquid during normal operation so that they are liquid sealed. Separated liquid is normally withdrawn from the vessel in this liquid collection space.
It is an object of the present invention to provide a vertical separation vessel with vane-inlet device, wherein the accumulation of liquid in the liquid collection space is improved.